There exists a need for a relatively economical form of metering dispenser that can be used with a variety of fluids and that can be permanently located or incorporated in the fluid container but that also allows the container to be provided with a positive means of closure, e.g to protect against accidental spillage. Dispensers in the form of optics are known that incorporate a non-return valve but these are relatively expensive in construction and would be uneconomic to incorporate them in containers that are intended to be disposed of when the contents have been consumed.
Simpler metering dispensers are known but they suffer of disadvantages. In GB 1224927 and U.S. Pat. No. 3005578 there is provided a metering chamber which is filled by inverting the container, but the container must then be held inverted while the user rotates the metering device to seal the chamber off from the main body of the fluid, which can be an extremely inconvenient operation, especially with larger containers. U.S. Pat. No. 3006506 has a similar arrangement, but with a permanently inverted container, which therefore has only a very limited application. DE 3133835 describes a metering device which requires the container to be held inverted and, even more inconveniently a plunger of the device must be pushed upwards while the container is held there.
In GB 2133774, when the container is inverted a metering chamber is filled by a tube that extends to the bottom of the container and is in permanent communication with the chamber, but it is then possible to dispense an excess quantity if more fluid is allowed to travel through the tube into the chamber when the container is tipped to dispense the fluid in the chamber. There are also known dispensing containers which generally do not permit a given volume of liquid to be automatically measured but require manipulation by the user to control the quantity being dispensed - as exemplified by GB 2093804 and U.S. Pat. No. 4214679.
In U.S. Pat. No. 3402860, metering dispensers are described which require the container to be squeezed to force the fluid into a metering chamber against the pressure of the air in the chamber. Repeated squeezing is needed to bring the liquid to the required level in the chamber as air must be allowed to escape from the chamber between squeezes. This introduces a source of error as it cannot be expected that the general user will always squeeze sufficiently hard and often to meter the intended quantity, but in addition the main volume of the container remains in communication with the metering chamber when the dose is dispensed and, as is admitted in U.S. Pat. No. 3402860, this is a source of potential inaccuracy that can only be minimized by the user observing a strict procedure.
Finally there can be mentioned DE 3417001 which describes a metering mechanism intended to prevent overdosing. To this end, a metering cup is axially slidable in the neck of a container to open and close connecting apertures between it and the main volume of the container. The apertures are disposed intermediate the height of a metering chamber and the dose metered by inverting the container to fill the normally uppermost region and then returning the container to the upright position. The cup is engaged by a screw closure cap which draw the cup up to close the apertures as the cap is removed, and the metered dose is hen dispensed. It is difficult, however, to produce such an arrangement that is easy to use and yet not prone to leakage: the axial engagement and disengagement of the cap with the cup requires some force as an interference fit is involved, which can make it difficult for use by the aged and infirm. If the cup is designed so as to slide too easily, not only is it difficult to seal the closure adequately but the position of the cup is easily disturbed if the container is shaken when tipped, which can resulting the apertures reopening and allowing excess fluid to pour through. Moreover, DE 3417001 proposes, for precise dosing, that the metering chamber space above the apertures should correspond to the space below the apertures but such an arrangement is prone to inaccuracy, firstly because it relies on the ability to locate the cap at a precise and repeatable axial position each time the metering mechanism is used, and further because it makes no allowance for the unskilled, domestic user who may allow some of the contents to spill back from the metering chamber when turning the container upright again and may affect the volume of the upper space by screwing the cap on more or less loosely.